The use of frequency encoded, decimal digit dialing in the telephone system requires a receiving circuit that is capable of detecting the tone signals originating at the subscriber's telephone. Such a receiver typically includes a tone detector capable of meeting two essential requirements. On one hand, it must be sufficiently sensitive to recognize the tone signals. On the other hand, it must be capable of discriminating between spurious signals, such as noise and speech, and valid dialing tones. These detector characteristics represent, to a large extent, conflicting requirements and, hence, any practical solution must involve a compromise between the ideal fulfillment of each. One approach, as described in an article by G. T. Kraemer and L. C. J. Roscoe, entitled "The Family of Touch-Tone Receivers," published in the 1965 July/August issue of the Bell Laboratories Record, Vol. 43, No. 7, pp. 282-287, employs the so-called "limiter guard action," whose operation is based upon the fact that an amplitude-limited signal contains a fixed amount of energy. Thus, an amplitude-limited spurious signal, containing many frequency components, will not contain sufficient energy at any one frequency to exceed a predetermined "critical threshold." A valid dial tone, on the other hand, has all its energy concentrated at the frequency of interest and, hence, readily exceeds the critical threshold. To increase the discriminator sensitivity, feedback to the limiter input is also employed. The net effect, however, is to limit the dynamic range of the receiver.
In an alternate approach, a separate voice filter and detector are provided which serve to disable the receiver whenever the voice channel signal exceeds a predetermined level. If made too sensitive, however, receiver noise can cause the voice channel to disable the receiver in the presence of a valid digit tone signal.